1. Field of the Invention
This invention relates to an apparatus for controlling the air-fuel ratio of an internal combustion engine.
2. Prior Art
Devices for detecting the air-fuel ratio of an internal combustion engine have previously been known. Such devices comprise a wide range air-fuel ratio sensor including an oxygen sensor section which generates an electromotive force in response to the difference between atmospheric pressure and the oxygen concentration in exhaust gas discharged from the engine, and an oxygen pump section which provides a pump-driving current to feed and discharge oxygen to and from the exhaust gas used for comparison with the atmospheric pressure, the flow of the pump-driving current being controlled so that the output voltage of the oxygen sensor attains a predetermined value, whereby the air-fuel ratio of the engine is detected with the magnitude of the pump-driving current (see, for example, Japanese Utility Model Public Disclosure No. 18659/1987). Thus, the control of the air-fuel ratio of the engine is carried out by using such a device to detect the air-fuel ratio. The above-mentioned device can continuously measure the air-fuel ratio over a wide range from rich to lean.
In a conventional apparatus for controlling air-fuel ratio, as mentioned above, the pump-driving current is fed so that the output voltage of the oxygen sensor section is kept constant, and the air-fuel ratio is detected by measuring the pump-driving current. If any trouble or failure occurs in the oxygen sensor section, the oxygen pump section, or a connector or the like associated therewith, the precise pump-driving current does not flow, even when the apparatus operates to establish a flow of the pump-driving current sufficient to cause the voltage of the oxygen sensor section to reach the predetermined value. Alternatively, if the current is able to flow normally, it may possibly happen that an excessive current flows as a result of insufficient electromotive force in the oxygen pump section, and thus correct information on the air-fuel ratio cannot be obtained. This has led to the problem that, if the feedback control of the air-fuel ratio is conducted in response to the operation of the wide range air-fuel ratio sensor when the latter is malfunctioning, the air-fuel ratio will be greatly affected such as to produce an excessively rich or lean mixture, resulting in poor performance, deterioration of the exhaust gas quality and possibly even stalling of the engine.